The present invention relates to a flash illumination system and more particularly to a flash lamp power supply which provides energy to the lamp in incremental amounts until a desired exposure level is reached.
When an object is reproduced, either photographically or elecrtrographically, by flash exposure of a light sensitive material, the density of the object (dark and light highlights) must be determined in some manner so that compensation can be made to the material being exposed to maintain uniformity. Prior art efforts to control exposure automatically are reflected in numerous publications. In photography applications, an automatic exposure control for an electronic flash unit may include a light meter and electronic circuitry which detects the instantaneous amount of light reflected from the scene to be photographed onto the light sensitive element of the light meter. The light meter contains circuitry which integrates the signal derived from the light detecting element. When the integrated signals reach a predetermined level corresponding to required film exposure the flash is extinguished.
The power to a flash unit is typically provided by charging a capacitor or series of capacitors to a desired voltage; the capacitor(s) are then discharged through the lamp and associated discharge circuitry creating the flash illumination.
Various ways are known to change exposure levels in response to changes in object density. In one method, the charging voltage applied to the lamp is readjusted in response to pre-sensing of object density. Another method relies on quenching the lamp when the desired exposure level is reached. This quenching can be accomplished with a commutating circuit which uses a series switch (SCR) which when opened, causes the lamp to extinguish. Alternatively, in a crowbar circuit, the SCR is connected in parallel with the lamp and, when closed, shunts current around the lamp. Using these circuits, a significant amount of energy must be used to accomplish the control. Also, the switch must be capable of fast turn-on, turn-off time, a characteristic that adds to the cost of the system. In co-pending application, USSN No. 077,232 filed on Sept. 19, 1979 assigned to the same assignee as the present invention, a variable output power supply is disclosed which uses circuitry which pre-senses, or computes during the flash interval, the precise amount of energy which must be supplied to the flash lamp to obtain the desired exposure level.